1. Field of the Invention
The present invention relates to a liquid crystal display device, an optical film for use in the liquid crystal display device, and a terminal device incorporating the liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device with excellent oblique viewing angle characteristics, an optical film for use in the liquid crystal display device, and a terminal device incorporating the liquid crystal display device.
2. Description of the Related Art
Display devices which employ liquid crystals, which are low in profile, light in weight, small in size, and of low power requirements, have recently been used in a wide variety of terminal devices including large-size terminal devices such as monitors, television sets, etc., medium-size terminal devices such as notebook personal computers, cash dispensers, automatic dispensing machines, etc., and small-size terminal devices such as personal television sets, PDAs (Personal Digital Assistants), mobile phones, portable game machines, etc.
The display devices which employ liquid crystals need to use a light source to enable the user to view displayed information because the liquid crystal molecules do not emit light themselves. Generally, liquid crystal display devices are classified into a transmissive type, a reflective type, and a semitransmissive type which uses transmitted light and reflected light in combination. The reflective liquid crystal display devices can be of low power requirements because they use external light for display. However, since the reflective liquid crystal display devices have poorer display properties such as contrast ratio than the transmissive liquid crystal display devices, the transmissive liquid crystal display devices and the semitransmissive liquid crystal display devices are mainstream liquid crystal display devices at present. The transmissive liquid crystal display devices and the semitransmissive liquid crystal display devices have a light source unit disposed behind the liquid crystal panel for displaying information based on light emitted by the light source unit. In particular, since the medium- and small-size liquid crystal display devices are carried around and used in various environments by the users, they are constructed as semitransmissive liquid crystal display devices which provide high image visibility by allowing the users to see reflective displayed images in bright places and transmissive displayed images in dark places.
Liquid crystal panels used in the semitransmissive liquid crystal display devices have been used in an ECB (Electrically Controlled Birefringence) mode and a multidomain vertical alignment mode that enables higher image quality and wider viewing angles. Recently, there have been proposed attempts to apply a lateral electric field mode which provides a wide viewing angle, in principle, to semitransmissive liquid crystal display devices.
An Nz coefficient and the angle dependence of a retardation which are used in the description of the present invention will be described below. The Nz coefficient is defined as:Nz=(nx−nz)/(nx−ny)  (1)where nx represents the refractive index in a direction (slow axis) exhibiting the maximum refractive index within the film plane of the birefringent medium of a retardation plate, ny the refractive index in an in-plane direction perpendicular to the direction, and nz the refractive index in a thickness-wise direction.
A retardation Re(0) with respect to light from the direction normal to the retardation plate is determined by the difference between the refractive indexes nx, ny in the main-axis direction within the plane and the thickness d of the retardation plate, as expressed by the following equation (2):Re(0)=(nx−ny)×d  (2)
Light at an angle from the normal direction, as viewed obliquely to the retardation plate, is affected not only by the retardation determined by nx, ny only, but also by the main-axis refractive index nz in the thickness-wise direction and an increased optical path provided by the inclination from the normal direction. In the present description, the retardation of light from the direction normal to the film plane is simply referred to as a retardation or Re(0), and the retardation of light traveling from air into the retardation plate at an angle of θ from the normal direction to the film plane is referred to as an oblique-view retardation or Re(θ). In particular, if the inclination θ is in a direction toward the slow axis, then the retardation is referred to as Rex(θ), and if the inclination θ is in a direction in a plane perpendicular to the slow axis, the retardation is referred to as Rey(θ).
With respect to a retardation plate used in a liquid crystal display device, it is known that the relationship between Re(θ), Rex(θ), and Rey(θ) is dependent on the Nz coefficient as follows: With respect to a retardation plate where Nz=1, nz=ny according to equation (1), and the index ellipsoid is in the shape of a rugby ball which is longest along the slow axis when observed from the direction normal to the substrate. At this time, Re(0)>Rex(θ) and Re(0)<Rey(θ).
With respect to a retardation plate where Nz=0, nz=nx according to equation (1), and the index ellipsoid is of a shape which is longest along the slow axis and thickness-wise direction. At this time, Re(0)<Rex(θ) and Re(0)>Rey(θ).
With respect to a retardation plate where Nz=0.5, nz=(nx+ny)/2 and the index ellipsoid is of a shape intermediate between the shape having retardation plate where Nz=1 and the shape with the retardation plate where Nz=0. When the inclination θ is in a direction toward the slow axis, Re(0)≈Rex(θ) and Re(0)≈Rey(θ) in a wide range of 0.
FIG. 1 is a schematic perspective view of a lateral-electric-field semitransmissive liquid crystal display device disclosed in JP2005-106967A, which is relevant to the present invention. In FIG. 1, an XYZ orthogonal coordinate system is established as follows: A direction from liquid crystal layer 2006a toward polarizer 2002 is defined as a +Z direction and an opposite direction as a −Z direction. Lateral directions in FIG. 1 are defined as X-axis directions, particularly a rightward direction as a +X direction and a leftward direction as a −X direction. A +Y direction is represented by a direction according to the right-handed coordinate system. Specifically, when the thumb of the right hand of a person points in the +X direction and the index finger points in the +Y direction, then the middle finger points in the +Z direction. In the schematic perspective view of the liquid crystal display device shown in FIG. 1, the liquid crystal display device comprises a polarizer, a retardation plate, liquid crystal layers, and a reflector. The disposition angle of the retardation plate is expressed as an angle formed between the slow axis of the retardation plate and the X-axis. A counterclockwise direction is defined as positive. Similarly, the disposition angle of the polarizer is expressed as an angle formed between the absorption axis of the polarizer and the X-axis. Similarly, the disposition angle of the liquid crystal layers which are horizontally oriented is expressed as an angle formed between the orientation axis of the liquid crystal layers and the X-axis when no voltage is applied to the liquid crystal.
Liquid crystal layers 2006a, 2006b comprise two regions including a transmissive region and a reflective region having a reflector. A transmissive display area of the liquid crystal display device as viewed from a display surface comprises polarizer 2002, half-wavelength plate 2005 in which Nz=1, liquid crystal layer 2006a, retardation plate 2004 in which Nz=0, half-wavelength plate 2003 in which Nz=0, and polarizer 2001 which are successively arranged in the order named. A reflective display area of the liquid crystal display device as viewed from the display surface comprises polarizer 2002, half-wavelength plate 2005 in which Nz=1, liquid crystal layer 2006b, and reflector 2007 which are successively arranged in the order named.
Liquid crystal layers 2006a, 2006b in the reflective display area and the transmissive display area are horizontally oriented. The retardation of liquid crystal layer 2006b in the reflective display area is represented by a quarter wavelength, and the retardation of liquid crystal layer 2006a in the transmissive display area is represented by a value that is slightly smaller than 2 times, i.e., 1.7 to 1.9 times, the retardation in the reflective display area. If one wavelength is 550 nm, then the retardation of liquid crystal layer 2006a in the trans-missive display area is set to a value ranging from 233.8 nm to 261.2 nm.
The disposition angle of polarizer 2002 is 90 degrees. The disposition angle of half-wavelength plate 2005 in which Nz=1 is 15 degrees. The disposition angle of liquid crystal layers 2006a, 2006b is 75 degrees. The disposition angle of retardation plate 2004 in which Nz=0 is 165 degrees. The disposition angle of half-wavelength plate 2003 in which Nz=0 is 105 degrees. The disposition angle of polarizer 2001 is 0 degree. Polarizer 2002, half-wavelength plate 2005 in which Nz=1, and liquid crystal layer 2006b in the reflective display area jointly make up a circular polarizer in a wide range of visible wavelengths (referred to as a wide-range circular polarizer).
Retardation plate 2004 in which Nz=0, half-wavelength plate 2003 in which Nz=0, and polarizer 2001 are paired respectively with liquid crystal layer 2006a in the transmissive display area, half-wavelength plate 2005 in which Nz=1, and polarizer 2002. Specifically, the retardation of retardation plate 2004 in which Nz=0 is the same as the retardation of liquid crystal layer 2006a in the transmissive display area, and the slow axis of retardation plate 2004 in which Nz=0 is perpendicular to the oriented direction of liquid crystal layer 2006a in the transmissive display area. Accordingly, the retardation of retardation plate 2004 in which Nz=0 and the retardation of liquid crystal layer 2006a in the transmissive display area cancel each other out.
If a birefringent medium in which Nz=1 and a birefringent medium in which Nz=0 are arranged with their slow axes being perpendicular to each other in the direction normal to the substrate, then the slow axes of the birefringent medium in which Nz=1 and the birefringent medium in which Nz=0 are kept perpendicular to each other in all visual directions. Since their retardations cancel each other out in a wide viewing angle range, the viewing angle is increased.
Since liquid crystal layer 2006a in the transmissive display area is horizontally oriented and the nematic liquid crystal is represented by nx>ny=nz, liquid crystal layer 2006a in the transmissive display area has Nz=1. Consequently, the retardations of retardation plate 2004 in which Nz=0 and liquid crystal layer 2006a in the transmissive display area cancel each other out not only in the direction normal to the substrate, but also in a wide viewing angle range.
The retardation of half-wavelength plate 2003 in which Nz=0 is the same as the retardation of half-wavelength plate 2005 in which Nz=1 which is paired therewith, and the slow axis of half-wavelength plate 2003 in which Nz=0 is perpendicular to the slow axis of half-wavelength plate 2005 in which Nz=1. Consequently, the retardations of half-wavelength plate 2003 in which Nz=0 and half-wavelength plate 2005 in which Nz=1 cancel each other out in a wide viewing angle range.
When no voltage is applied to the liquid crystal of the lateral-electric-field semitransmissive liquid crystal display device disclosed in JP2005-106967A, light that has passed through polarizer 2002, half-wavelength plate 2005 in which Nz=1, and liquid crystal layer 2006b in the reflective display area, which jointly make up the circular polarizer, is circularly polarized. The light is converted into opposite circularly polarized light when it is reflected by reflector 2007, and cannot pass through the circular polarizer. Therefore, the reflective display area displays black when no voltage is applied. When voltage is applied to the liquid crystal, since the birefringence of the liquid crystal layer in the reflective display area is changed, polarizer 2002, half-wavelength plate 2005 in which Nz=1, and liquid crystal layer 2006b in the reflective display area do not serve as a circular polarizer, and white is displayed as the light is emitted therethrough. Accordingly, the liquid crystal display device operates as a normally black reflective display device.
In the transmissive display area when no voltage is applied to the liquid crystal, as the retardations of the two birefringent mediums that are present between polarizer 2001 and polarizer 2002 cancel each other out in a wide viewing angle range, the mediums between polarizer 2001 and polarizer 2002 provide a phase which is nearly isotropic. Since the transmission axes of polarizer 2001 and polarizer 2002 are perpendicular to each other, the transmissive display area will ideally displays black. When voltage is applied to the liquid crystal, since the birefringence of liquid crystal layer 2006a in the trans-missive display area is changed, the light is emitted therethrough, displaying white. Accordingly, the liquid crystal display device operates as a normally black transmissive display device.
JP2005-106967A states that a half-wavelength plate in which Nz=1 may be combined instead of half-wavelength plate 2003 in which Nz=0 and a half-wavelength plate in which Nz=0 may be combined instead of half-wavelength plate 2005 in which Nz=1.
As described above, the semitransmissive liquid crystal display device wherein the lateral electric field mode electrodes serve as the reflective display area and the mediums between the lateral electric field mode electrodes serve as the transmissive display area is disclosed in JP2005-106967A.
However, the liquid crystal display device described above is problematic in that the viewing angle characteristics of the reflective display area are not increased, and also in that it is highly costly to manufacture because the retardation plates of types having increased characteristics are used. Furthermore, since the types of the paired retardation plates are different from each other, the contrast ratio of the transmissive display is possibly reduced. These three problems will be described in detail below.
The viewing angle characteristics of a liquid crystal display device whose reflective display area comprises a circular polarizer are determined by the viewing angle characteristics of the circular polarizer. In the above liquid crystal display device, polarizer 2002, half-wavelength plate 2005 in which Nz=1 (or a half-wavelength plate in which Nz=0), and liquid crystal layer 2006b in the reflective display area jointly make up a wide-range circular polarizer. With respect to light from the normal direction to the substrate, they function as a wide-range circular polarizer to reduce the leakage of light to enable higher contrast ratio when black is displayed. With respect to light at an angle inclined from the normal direction, i.e., as light is obliquely viewed, it is affected by the main-axis refractive index nz in the thickness-wise direction and an increased optical path provided by the inclination from the normal. Therefore, when the light is obliquely viewed, half-wavelength plate 2005 in which Nz=1 (or a half-wavelength plate in which Nz=0) and liquid crystal layer 2006b in the reflective display area deviate from the design values of the wide-range circular polarizer, resulting in an increased leakage of light when black is displayed and a reduction in the contrast ratio. Consequently, the above liquid crystal display device is disadvantageous in that the viewing angle characteristics of the reflective display area are not increased.
The retardation plates of the above liquid crystal display device include half-wavelength plate 2003 in which Nz=0, retardation plate 2004 in which Nz=0 which is designed to match the retardation ranging from 233.8 nm to 261.2 nm of liquid crystal layer 2006a, and half-wavelength plate 2005 in which Nz=1. Accordingly, the liquid crystal display device requires three retardation plates of three types, and hence is costly to manufacture.
Half-wavelength plate 2003 in which Nz=0 and half-wavelength plate 2005 in which Nz=1 need to be paired with each other to cancel out their retardations in order to prevent the contrast ratio of the transmissive display from being lowered. For example, polystyrene is known as the material of a retardation plate in which Nz=0 and polycarbonate as the material of a retardation plate in which Nz=1. Thus, the materials of a retardation plate in which Nz=0 and a retardation plate in which Nz=1 are different from each other. Therefore, half-wavelength plate 2003 in which Nz=0 and half-wavelength plate 2005 in which Nz=1 have different refractive index wavelength dispersions and have their retardations not sufficiently canceled. If the retardations Re(0) of half-wavelength plate 2003 in which Nz=0 and half-wavelength plate 2005 in which Nz=1 are not sufficiently canceled out, then the contrast ratio of the transmissive display is lowered.